Vacation replacement / surrogate

From time to time, we run into the necessity to name surrogates for several roles during vacational or other leaves.

Inside circles, this is no problems, because it's easily communicated, but some roles are also explicitly there to be contacted from other circles and the information on who is the current surrogate would be great, so that the role doesn't need to be fully reassigned for only a few days.

So the concrete feature request would be:

For each role filler of a role, add the possibility to name a surrogate for a limited time (specifying start and end date). Holaspirit should show the surrogate during that time period instead of the role filler itself.